Best comment on reddit that I've found so far: "I think that yes the grand jury saw actual evidence, and heard witnesses, both from the government and an independent coroner report, but grand jury proceedings are very subjective. The prosecutor has almost complete discretion on what evidence to present and has no burden to include exculpatory evidence. He can use hearsay, and some news reports indicate witnesses testified with only hearsay statements. It is just a very difficult process to understand without having seen it, and it confuses a lot of people (as all this debate proves).
I think there was enough evidence to take the case to trial, maybe not for murder but possibly manslaughter. I think a trial would have allowed public disclosure of facts, and also helped ease some tension (but that is a long shot). And grand juries are very hesitant to indict police officers and prosecutors, who are essentially colleagues of officers, often can be perceived to have lead the jury in that direction.
But at the end of the day, there was no indictment, and I doubt they re convene another grand jury to seek one. http://www.reddit.com/r/funny/comments/2ndnpo/with_the_events_unfolding_in_ferguson_this_just_in/cmcurvi